U.S. patent application number 09/921562 was filed with the patent office on 2002-01-10 for optical device.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Saito, Kazuhito, Sano, Tomomi, ichiro Takahashi, Ken?apos, Tamura, Mitsuaki.
Application Number | 20020003927 09/921562 |
Document ID | / |
Family ID | 18730365 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020003927 |
Kind Code |
A1 |
Tamura, Mitsuaki ; et
al. |
January 10, 2002 |
Optical device
Abstract
An object of the present invention is to provide an optical
device that can restrain the dependence of the center wavelength of
reflection at the diffraction grating upon the voltage applied to
the piezoelectric element from shifting depending on the
temperature. An embodiment of the optical device has the following
structure. Optical device 1 has a U-shaped member 2, an optical
fiber 4 having a diffraction grating portion 8 formed therein and a
piezoelectric element (PZT based ceramics) 6. The U-shaped member 2
has a bottom portion 2a and a pair of arm portions 2b and 2c
extending from the bottom portion 2a The U-shaped member consists
of a material (aluminum alloy or the like) having a thermal
expansion coefficient greater than the piezoelectric element 6. A
voltage applying means 10 for applying a voltage is connected to
the piezoelectric element 6, and the amount of displacement of the
piezoelectric element 6 changes according to the magnitude of the
applied voltage. The piezoelectric element 6 is in the form of a
rod and is fixed to the U-shaped member 2 so as to be connected to
the respective intermediate points of the arm portions 2b and 2c.
The ends of the arm portions 2b and 2c of the U-shaped member 2 are
fixed to the optical fiber 4 in such a manner as to stride the
diffraction grating portion 8 formed in the optical fiber.
Inventors: |
Tamura, Mitsuaki;
(Yokohama-shi, JP) ; Saito, Kazuhito;
(Yokohama-shi, JP) ; Sano, Tomomi; (Yokohama-shi,
JP) ; Takahashi, Ken?apos;ichiro; (Yokohama-shi,
JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
|
Family ID: |
18730365 |
Appl. No.: |
09/921562 |
Filed: |
August 6, 2001 |
Current U.S.
Class: |
385/37 ; 385/136;
385/27 |
Current CPC
Class: |
G02B 6/29317 20130101;
G02B 6/0218 20130101; G02B 6/022 20130101 |
Class at
Publication: |
385/37 ; 385/27;
385/136 |
International
Class: |
G02B 006/34 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 7, 2000 |
JP |
238702/2000 |
Claims
What is claimed is:
1. An optical device comprising: an optical fiber having a given
length of diffraction grating formed in the direction of the
optical axis thereof; a rod-shaped piezoelectric element; a means
for applying a voltage to said piezoelectric element; and a
U-shaped member having a pair of arm portions, said optical fiber
being fixed to a pair of ends of said arm portions such that the
diffraction grating of said optical fiber is positioned between the
pair of ends of said arm portions as if the ends of said arm
portions stride the diffraction grating portion, said piezoelectric
element being fixed to said U-shaped member such that said
piezoelectric element is connected to the pair of said arm portions
at their intermediate points, and said U-shaped member being made
of a material having a larger thermal expansion coefficient than
that of said piezoelectric element.
2. An optical device according to claim 1, wherein said U-shaped
member is made of stainless steel.
3. An optical device according to claim 1, wherein said U-shaped
member is made of aluminum alloy.
4. An optical device comprising: an optical fiber having a given
length of diffraction grating formed in the direction of the
optical axis thereof, a rod-shaped piezoelectric element, a means
for applying a voltage to said piezoelectric element, a rod-shaped
member, first members constituting a pair of arm parts, and second
members adhered to said first members; said optical fiber being
fixed to the ends of said pair of arm parts of said first members
such that the diffraction grating of said optical fiber is
positioned between the ends of said pair of arm parts of said first
members, said rod-shaped member being fixed to the other ends
(i.e., opposite the ends to which said optical fiber is fixed) of
said first members, said rod-shaped member and said pair of arm
parts of said first members constituting a U-shaped member, said
piezoelectric element being fixed to said first members so as to be
connected to said pair of arm parts at their intermediate points,
said second members being adhered to the first members
longitudinally on the side opposite to the side to which both said
piezoelectric element and said rod-shaped member are fixed; wherein
said rod-shaped member and said piezoelectric element have a
substantially equal thermal expansion coefficient, and said second
members are made of a material having a thermal expansion
coefficient that is larger than that of said first members.
5. An optical device according to claim 4, wherein said first
members are made of Invar alloy.
6. An optical device according to claim 4, wherein said first
members are ceramic.
7. An optical device according to claim 4, wherein said second
members are made of aluminum alloy.
8. An optical device according to claim 4, wherein said rod-shaped
member is a piezoelectric element.
9. An optical device comprising: an optical fiber having a given
length of diffraction grating formed in the direction of the
optical axis, a rod-shaped piezoelectric element, a means for
applying a voltage to said piezoelectric element, a rod-shaped
member, first members constituting a pair of arm parts, and second
members adhered to the first members; said optical fiber being
fixed to the ends of said pair of arm parts of said first members
such that the diffraction grating of said optical fiber is
positioned between the ends of said pair of arm parts of said first
members, said rod-shaped member being fixed to the other ends
(i.e., opposite the ends to which said optical fiber is fixed) of
said first members, said rod-shaped member and said pair of arm
parts of said first members constituting a U-shaped member, said
piezoelectric element being fixed to said first members so as to be
connected to said pair of arm parts at their intermediate points,
said second members being adhered to the first members
longitudinally on the side to which both said piezoelectric element
and said rod-shaped member are fixed; wherein said rod-shaped
member and said piezoelectric element have a substantially equal
thermal expansion coefficient, and said second members are made of
a material having a thermal expansion coefficient that is lower
than that of said first members.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical device equipped
with an optical fiber having a given length of diffraction grating
along its optical axis.
[0003] 2. Description of the Background
[0004] A known example of this kind of optical device is the
optical device disclosed in the gazette of Japanese Patent
Application Laid-Open No. 10-206753. The optical device disclosed
in Japanese Patent Application Laid-Open No. 10-206753 has an
optical fiber having a grating portion (diffraction grating) and a
piezoelectric element that affords a stress to the grating portion.
The ends of the piezoelectric element are mechanically secured to
both ends of the grating portion through blocks. Applying a voltage
to the piezoelectric element causes a displacement of the
piezoelectric element in the longitudinal direction, and this
displacement is transferred to the grating portion through the
blocks. It is possible to alter the displacement of the
piezoelectric element by changing the applied voltage. Therefore,
if the displacement of longitudinal direction caused to the
piezoelectric element due to the applied voltage is transferred to
the grating portion through the blocks, the center wavelength of
reflection at the grating portion can be changed.
[0005] However, there is the following problem in the optical
device of the above structure. In the diffraction grating, because
the optical fiber has a positive thermal expansion coefficient, the
optical fiber expands or shrinks according to the variation of the
ambient temperature, thereby causing the grating pitch to change.
Also, the refractive index in the glass portion of the optical
fiber changes according to the variation of temperature. As a
result, the center wavelength of reflection at the diffraction
grating has temperature dependence. If the center wavelength of
reflection at the diffraction grating has temperature dependence,
there occurs a phenomenon in which the dependence of the center
wavelength of reflection at the diffraction grating upon the
voltage applied to the piezoelectric element shifts to the long
wavelength side or the short wavelength side, depending on the
temperature as shown in FIG. 10. For example, the characteristic A
exhibited at 20.degree. C. is shifted to the long wavelength side
as the characteristic B at 70.degree. C. by the expansion of the
piezoelectric element. On the other hand, at -20.degree. C., it is
shifted to the short wavelength side as the characteristic C by the
shrinkage of the piezoelectric element.
SUMMARY OF THE INVENTION
[0006] In view of the above-mentioned problem the present invention
is made for the purpose of providing an optical device that can
restrain the dependence of the center wavelength of reflection at
the diffraction grating upon the voltage applied to the
piezoelectric element from being shifted by the variation of
temperature. The optical device according to the present invention
is obtained by combining various materials having a suitable
thermal expansion coefficient.
[0007] An optical device according to a first embodiment of the
present invention comprises an optical fiber having a given length
of diffraction grating formed in the direction of the optical axis,
a rod-shaped piezoelectric element, a means for applying voltage on
the piezoelectric element, and a U-shaped member having a pair of
arm portions. The optical fiber is fixed to a pair of ends of the
arm portions such that the diffraction grating of the optical fiber
is positioned between the pair of ends of the arm portions. The
piezoelectric element is fixed to the U-shaped member such that the
piezoelectric element is connected to the pair of arm portions at
their intermediate points. The U-shaped member is made of a
material having a thermal expansion coefficient that is larger than
that of the piezoelectric element.
[0008] In the optical device according to the first embodiment of
the present invention, since the U-shaped member is made of a
material having a thermal expansion coefficient that is larger than
that of the piezoelectric element, the shrinkage amount of the
bottom portion of the U-shaped member is larger than the shrinkage
amount of the piezoelectric element at low temperature.
Consequently, the interval between the ends of the arm portions of
the U-shaped member expands because the points where the
piezoelectric element is fixed perform as a fulcrum. On the other
hand, the expansion amount of the bottom portion of the U-shaped
member is larger than the expansion amount of the piezoelectric
element at high temperature. Consequently, the interval between the
ends of the arm portions of the U-shaped member narrows because the
points where the piezoelectric element is fixed perform as a
fulcrum. Thus, the interval between the ends of the arm portions of
the U-shaped member changes as if the U-shaped member had a
negative thermal expansion coefficient. As a result, it is possible
to restrain the dependence of the center wavelength of reflection
at the diffraction grating upon the voltage applied to the
piezoelectric element from being shifted to the long wavelength
side or the short wavelength side depending on the temperatures.
Also, the displacement caused to the piezoelectric element can be
transferred to the diffraction grating after being enlarged by the
U-shaped member.
[0009] Preferably, the U-shaped member is made of stainless steel.
The stainless steel is suitable for transferring the displacement
of the piezoelectric element to the diffraction grating efficiently
because it has high elasticity and its thermal expansion
coefficient is larger than that of the piezoelectric element.
[0010] More preferably, the U-shaped member is made of aluminum
alloy. Since the thermal expansion coefficient of aluminum alloy is
far larger than that of the piezoelectric element, the displacement
of the piezoelectric element can be more efficiently transferred to
the diffraction grating, and the optical device can be
miniaturized.
[0011] An optical device according to a second embodiment of the
present invention comprises an optical fiber having a given length
of diffraction grating formed in the direction of the optical axis,
a rod-shaped piezoelectric element, a means for applying a voltage
to the piezoelectric element, a rod-shaped member, first members
constituting a pair of arm parts, and second members adhered to the
first members. The optical fiber is fixed to the ends of the pair
of arm parts of the first members such that the diffraction grating
of the optical fiber is positioned between the ends of the pair of
arm parts of the first members. The rod-shaped member is fixed to
the other ends (i.e., opposite the ends to which the optical fiber
is fixed) of the first members constituting a pair of arm parts
such that the rod-shaped member and the pair of arm parts of the
first members form a U-shaped member. The piezoelectric element is
fixed to the first members such that the piezoelectric element is
connected to the pair of arm parts at their intermediate points.
The rod-shaped member and the piezoelectric element have a
substantially equal thermal expansion coefficient. The second
members are made of a material having a thermal expansion
coefficient that is larger than that of the first members.
[0012] The second members are adhered to the first members
longitudinally on the opposite side (i.e., opposite to the side of
the first members constituting the pair of arm parts to which the
piezoelectric element and the rod-shaped member are fixed). The
thermal expansion coefficient of the second members is larger than
that of the first members.
[0013] In the optical device according to the second embodiment of
the present invention, the interval between the ends of the pair of
arm parts of the first members where the optical fiber is fixed
changes according to the variation of temperature in the following
manner.
[0014] When the temperature becomes low, the rod-shaped member and
the piezoelectric element shrink and tend to narrow the interval of
the pair of arm parts of the first members. However, since the
second members shrink more than the first members, the first
members are transformed into an arc-shape, in which the side where
each second member is attached is the inside of the arc.
Accordingly, the interval between the ends of the pair of arm parts
of the first members is enlarged.
[0015] On the other hand, when the temperature becomes high, the
rod-shaped member and the piezoelectric element expand and tend to
expand the interval of the pair of arm parts of the first members.
However, the second members expand more than the first members, and
the first members are transformed into an arc-shape, in which the
side where each second member is attached is the outside of the
arc. Accordingly, the interval between the ends of the pair of arm
parts of the first members is narrowed.
[0016] As described above, the interval between the ends of the
pair of arm parts of the first members changes as if the first
members had a negative thermal expansion coefficient. As a result,
it is possible to restrain the dependence of the center wavelength
of reflection at the diffraction grating upon the voltage applied
to the piezoelectric element from being shifted to the long
wavelength side or the short wavelength side according to the
temperature. Also, the displacement caused to the piezoelectric
element can be enlarged and transferred to the diffraction grating
by the first members and the rod-shaped member.
[0017] Moreover, since the mechanism for enabling the first members
to perform as if virtually having a negative thermal expansion
coefficient and the mechanism for expanding the displacement of the
piezoelectric element are independent of each other, a degree of
freedom for setting the variation to be given to the center
wavelength of reflection at the grating portion is increased.
Consequently, the variable range of the center wavelength of
reflection at the grating portion is enlarged.
[0018] Preferably, the first members are made of Invar alloy. The
thermal expansion coefficient of Invar alloy is low, and a design
can be made so that a large amount of warp of the first members may
be obtained.
[0019] It is also preferable that the first members are made of
ceramics. The thermal expansion coefficient of the ceramics is low,
and the first members can be designed to exhibit a large amount of
warp.
[0020] Preferably, the second members are made of aluminum alloy.
The thermal expansion coefficient of aluminum alloy is large, and a
large amount of warp of the first members can be designed.
[0021] The rod-shaped member preferably is made of a piezoelectric
element. Since the rod-shaped member is a piezoelectric element,
the thermal expansion coefficient of the rod-shaped member is the
same as the piezoelectric element.
[0022] An optical device according to a third embodiment of the
present invention comprises an optical fiber having a given length
of diffraction grating formed in the direction of the optical axis,
a rod-shaped piezoelectric element, a means for applying a voltage
to the piezoelectric element, a rod-shaped member, first members
constituting a pair of arm parts, and second members adhered to the
first members. The optical fiber is fixed to the ends of the pair
of arm parts of the first members such that the diffraction grating
of the optical fiber is positioned between the ends of the pair of
arm parts of the first members. The rod-shaped member is fixed to
the other ends (i.e., opposite the ends to which the optical fiber
is fixed) of the first members constituting a pair of arm parts
such that the rod-shaped member and the pair of arm parts of the
first members constitute a U-shaped member. The piezoelectric
element is fixed to the first members such that the piezoelectric
element is connected to the pair of arm parts at their intermediate
points. The rod-shaped member and the piezoelectric element have a
substantially equal thermal expansion coefficient. The second
members are adhered to the first members longitudinally on the side
of the first members constituting the pair of arm parts to which
the piezoelectric element and the rod-shaped member are fixed. The
thermal expansion coefficient of the second members is smaller than
that of the first members.
[0023] In the optical device according to the third embodiment of
the present invention, the interval between the ends of the pair of
arm parts of the first members where the optical fiber is fixed
changes according to the variation of temperature in the following
manner.
[0024] When the temperature becomes low, the rod-shaped member and
the piezoelectric element shrink and tend to narrow the interval of
the pair of arm parts of the first members. However, since the
first members shrink more than the second members, the first
members are transformed into an arc-shape, in which the side where
each second member is attached is the outside of the arc.
Accordingly, the interval between the ends of the pair of arm parts
of the first members is enlarged.
[0025] On the other hand, when the temperature becomes high, the
rod-shaped member and the piezoelectric element expand and tend to
expand the interval of the pair of arm parts of the first members.
However, the first members expand more than the second members, and
the first members are transformed into an arc-shape, in which the
side where each second member is attached is the inside of the arc.
Accordingly, the interval between the ends of the pair of arm parts
of the first members is narrowed.
[0026] As described above, the interval between the ends of the
pair of arm parts of the first members changes as if the first
members had a negative thermal expansion coefficient. As a result,
it is possible to restrain the dependence of the center wavelength
of reflection at the diffraction grating upon the voltage applied
to the piezoelectric element from being shifted to the long
wavelength side or the short wavelength side according to the
variation of temperature. Also, the displacement caused to the
piezoelectric element can be enlarged and transferred to the
diffraction grating by the first members and the rod-shaped
member.
[0027] Moreover, since the mechanism for enabling the first members
to perform as if virtually having a negative thermal expansion
coefficient and the mechanism for expanding the displacement of the
piezoelectric element are independent of each other, a degree of
freedom for setting the variation to be given to the center
wavelength of reflection at the grating portion is increased.
Consequently, the variable range of the center wavelength of
reflection at the grating portion is enlarged.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic diagram showing an optical device
according to the first embodiment of the present invention.
[0029] FIG. 2 illustrates the influence of temperature on the
optical device according to the first embodiment of the present
invention.
[0030] FIG. 2(a) shows a condition at low temperature, and
[0031] FIG. 2(b) shows a condition at high temperature.
[0032] FIG. 3 illustrates a state in which the displacement of the
piezoelectric element is enlarged and transferred to the
diffraction grating by the U-shaped member in the optical device
according to the first embodiment of the present invention.
[0033] FIG. 4 illustrates the operation of the optical device
according to the first embodiment of the present invention. FIG.
4(a) shows the condition in which no voltage is applied to the
piezoelectric element at normal temperature.
[0034] FIG. 4(b) shows the condition in which no voltage is applied
to the piezoelectric element at high temperature.
[0035] FIG. 4(c) shows the condition in which a voltage is applied
to the piezoelectric element at high temperature.
[0036] FIG. 5 is a chart showing the dependence of the center
wavelength of reflection at a diffraction grating upon the voltage
applied to the piezoelectric element in the optical device
according to the first embodiment of the present invention.
[0037] FIG. 6 is a schematic diagram showing an optical device
according to the second embodiment of the present invention.
[0038] FIG. 7 illustrates the influence of temperature on the
optical device according to the second embodiment of the present
invention.
[0039] FIG. 7(a) shows a condition at low temperature, and
[0040] FIG. 7(b) shows a condition at high temperature.
[0041] FIG. 8 illustrates a state in which the displacement of the
piezoelectric element is enlarged and transferred to the
diffraction grating in the optical device according to the second
embodiment of the present invention.
[0042] FIG. 9 illustrates the operation of the optical device
according to the second embodiment of the present invention.
[0043] FIG. 9(a) shows a condition in which no voltage is applied
to the piezoelectric element at normal temperature.
[0044] FIG. 9(b) shows a condition in which no voltage is applied
to the piezoelectric element at high temperature, and
[0045] FIG. 9(c) shows a condition in which a voltage is applied to
the piezoelectric element at high temperature.
[0046] FIG. 10 is a chart showing a state where the dependence of
the center wavelength of reflection at a diffraction grating upon
the voltage applied to the piezoelectric element is shifted by the
variation of temperature in a conventional optical device.
DETAILED DESCRIPTION
[0047] In the following, the preferred embodiments of the optical
device according to the present invention will be explained in
detail. With reference to the accompanying drawings, the same
reference marks denote the same parts, where possible, throughout
the drawings, and a repeated explanation will be omitted. The
dimensions in the drawings are partly exaggerated and do not always
correspond to actual ratios of dimensions.
[0048] The embodiments according to the respective embodiments of
the present invention are shown in the case of application to a
wavelength-variable optical device that is used for taking out a
light signal having a specific wavelength selectively from
wavelength-division multiplexed light signals in a multiplexed
optical network, for example.
[0049] (First Embodiment)
[0050] FIG. 1 is a schematic diagram showing an optical device
according to the first embodiment of the present invention. The
optical device 1 comprises a U-shaped member 2, an optical fiber 4
having a diffraction grating portion 8, and a piezoelectric element
6. The U-shaped member 2 includes a bottom portion 2a and a pair of
arm portions 2b and 2c extending from the bottom portion to the
optical fiber 4. The U-shaped member 2 is made of material having a
larger thermal expansion coefficient than that of the piezoelectric
element 6, such as stainless steel or aluminum alloy. The
piezoelectric element 6 can be made of PZT (lead zirconate
titanate) based ceramics, barium titanate based ceramics, lead
titanate based ceramics, or the like.
[0051] The piezoelectric element 6 is connected to a means 10 for
applying a voltage to the piezoelectric element 6 (hereinafter
occasionally, such means is referred to as a "voltage applying
means"). The piezoelectric element 6 changes in the amount of its
displacement according to the magnitude of voltage applied by the
voltage applying means 10. The piezoelectric element 6 is formed in
a rod-shape, and the ends of the piezoelectric element are fixed to
the U-shaped member 2 at the intermediate points of the pair of arm
portions 2b and 2c, respectively. Such fixation on the U-shaped
member 2 of piezoelectric element 6 is achieved by using a method
such as adhesion, weld, screw, caulking, or the like.
[0052] The ends of the arm portions 2b and 2c of the U-shaped
member 2 are fixed to the optical fiber 4 such that the diffraction
grating portion 8 of the optical fiber 4 is positioned between the
ends of the arm portions of the U-shaped member 2 as if the ends of
the arm portions stride the diffraction grating portion. The
diffraction grating portion 8 of the optical fiber 4 is formed
along the direction of the optical axis of the optical fiber 4 such
that at least the refractive index of the core portion changes. The
diffraction grating portion 8 can be formed, for example, by
irradiating ultraviolet rays to the core portion where a cladding
portion is exposed by removing a given length of a covering layer
provided around the cladding portion. The diffraction grating
portion 8 also can be formed without removing the covering layer
around the cladding layer. The irradiation of the ultraviolet rays
is performed by a known holographic method or phase grating method
or the like.
[0053] The optical fiber 4 is fixed by adhesion with an adhesive to
a pair of the ends 2b and 2c of the arm portions of the U-shaped
member 2. Specifically, the optical fiber 4 is fixed with an
adhesive at both sides of the diffraction grating portion 8 to the
ends of the arm portions 2b and 2c in a state such that a
predetermined tension is given to the diffraction grating portion 8
while the center wavelength of diffraction grating portion 8 is
monitored. When the optical fiber 4 is fixed in this manner, the
diffraction grating portion 8 of the optical fiber 4 and the
U-shaped member 2 are heated to a predetermined temperature: a
hardening temperature when the adhesive is a heat curable resin,
and when the adhesive is an ultraviolet ray curable resin, a
temperature at which the viscosity of the resin becomes
sufficiently low to give good wettability to the resin. By fitting
the optical fiber 4 to the U-shaped member 2 in this way, the
tension applied to the optical fiber 4 is maintained at a suitable
value in a operating temperature range (-45.degree. C. to
80.degree. C.) of the optical device 1. Consequently, it is
possible to maintain the stability of the center wavelength of
reflection against the temperature variation at the diffraction
grating portion 8.
[0054] In the optical device 1 having the above-mentioned
structure, since the U-shaped member 2 is made of material having a
thermal expansion coefficient which is larger than the
piezoelectric element 6, at a low temperature (e.g., 20.degree. C.)
the amount of shrinkage of the bottom portion 2a of the U-shaped
member 2 is larger than that of the piezoelectric element 6 as
shown in FIG. 2(a). Consequently, the interval (the distance
between the two points, where the optical fiber 4 is fixed at both
sides of the diffraction grating portion 8) between the ends of the
arm portions 2b and 2c of the U-shaped member 2 expands by the
function of a fulcrum at the points where the piezoelectric element
6 is fixed to the U-shaped member 2. On the other hand, at high
temperature (e.g., 70.degree. C.) the interval (the distance
between the two points, where the optical fiber 4 is fixed at both
sides of the diffraction grating portion 8) between the ends of the
arm portions 2b and 2c of the U-shaped member 2 narrows by the
function of a fulcrum at the points where the piezoelectric element
6 is fixed to the U-shaped member 2. That is because the amount of
the expansion of the bottom portion 2a of the U-shaped member 2 is
larger than that of the piezoelectric element 6 as shown in FIG.
2(b). The illustration of the voltage applying means 10 is omitted
in FIGS. 2(a) and (b).
[0055] In the optical device 1 having the above-mentioned
structure, when a voltage is applied to the piezoelectric element 6
from the voltage applying means 10, a displacement occurs in the
longitudinal direction of the piezoelectric element 6 according to
the applied voltage, and the displacement is enlarged by the
U-shaped member 2 as shown in FIG. 3, and is transferred to the
diffraction grating portion 8. The center wavelength of reflection
at the diffraction grating portion 8 can, therefore, be varied
efficiently according to the magnitude of voltage applied to the
piezoelectric element 6.
[0056] Thus, in the optical device 1, the temperature dependence of
the center wavelength of reflection can be substantially eliminated
because the tension applied to the optical fiber 4 is decreased,
and the center wavelength of reflection at the diffraction grating
portion 8 becomes approximately .lambda.a (nm) since the interval
between the ends 2b and 2c of the arm portions of the U-shaped
member 2 narrows to a predetermined value G2 (<G1) as shown in
FIG. 4(b) when the ambient temperature of the optical device 1
rises to 70.degree. C., for example, under the condition that the
interval between the ends 2b and 2c of the arm portions of the
U-shaped member 2 becomes a predetermined value G1 as shown in FIG.
4(a), and the center wavelength of reflection at the diffraction
grating portion 8 becomes .lambda.a (nm) when no voltage is applied
to the piezoelectric element 6 at normal temperature (e.g.,
20.degree. C.). Accordingly, under a condition (the condition shown
in FIG. 4(c)) in which a voltage is applied to the piezoelectric
element 6, the interval between the ends 2b and 2c of the arm
portions of the U-shaped member 2 changes only to an extent that
corresponds to the displacement of the piezoelectric element 6, and
the center wavelength of reflection at the diffraction grating
portion 8 changes only to an extent (.DELTA..lambda.) that
corresponds to the applied voltage and becomes
.lambda.a+.DELTA..lambda.(nm).
[0057] Also, the temperature dependence of the center wavelength of
reflection can be substantially eliminated because the tension
applied to the optical fiber 4 is increased, and the center
wavelength of reflection at the diffraction grating portion 8
becomes approximately .lambda.a (nm) since the interval between the
ends 2b and 2c of the arm portions of the U-shaped member 2 expands
as described above when the ambient temperature of the optical
device 1 decreases to -20.degree. C. from a normal temperature
(e.g., 20.degree. C.) under the condition that no voltage is
applied to the piezoelectric element 6. Accordingly, under the
condition in which a voltage is applied to the piezoelectric
element 6, the interval between the ends 2b and 2c of the arm
portions of the U-shaped member 2 changes only to an extent that
corresponds to the displacement of the piezoelectric element 6, and
the center wavelength of reflection at the diffraction grating
portion 8 changes only to an extent (.DELTA..lambda.) that
corresponds to the applied voltage and becomes
.lambda.a+.DELTA..lambda.(nm).
[0058] As described above, in the optical device 1 of the present
embodiment, the interval between the ends 2b and 2c of the arm
portions of the U-shaped member 2 changes as if the U-shaped member
2 apparently had a negative thermal expansion coefficient. As a
result, it is possible to restrain the dependence of the center
wavelength of reflection at the diffraction grating portion 8 upon
the voltage applied to the piezoelectric element from shifting to
the long wavelength side or the short wavelength side according to
the variation of temperature as shown in FIG. 5. FIG. 5 shows an
example in which the center wavelength of reflection at the
diffraction grating portion 8 is designed to be 1533.0 nm when the
applied voltage is 0 V, and the center wavelength of reflection at
the diffraction grating portion 8 changes to 1536.0 nm as the
applied voltage is altered to 120 V.
[0059] In the case where the U-shaped member 2 is made of stainless
steel, the displacement of the U-shaped member 2 can be transferred
efficiently to the diffraction grating portion 8 because the
stainless steel has a high elasticity as well as a larger thermal
expansion coefficient than that of the piezoelectric element.
[0060] Furthermore, when the U-shaped member 2 is made of aluminum
alloy, the displacement can be more efficiently transferred to the
diffraction grating portion 8 and the size of the optical device 1
as a whole can be reduced because the thermal expansion coefficient
of aluminum alloy is far larger than that of the piezoelectric
element 6.
[0061] (Second Embodiment)
[0062] FIG. 6 is a schematic diagram showing the optical device
according to the second embodiment of the present invention. An
optical device 21 comprises a pair of first members 22, second
members 23, an optical fiber 4 having the diffraction grating
portion 8, a piezoelectric element 6, and a rod-shaped member
25.
[0063] The first members 22 are made of a material having a given
thermal expansion coefficient, for example, Invar alloy or
ceramics. The first members 22 are fixed to both ends of the
piezoelectric element 6 and the ends of the rod-shaped member 25 in
a state in which piezoelectric element 6 and the rod-shaped member
25 are disposed in parallel. The respective fixation of the
piezoelectric element 6 and the rod-shaped member 25 to the first
members 22 is done by adhesion, welding, screwing, caulking,
etc.
[0064] The second members 23 are made of a material such as
aluminum alloy having a larger thermal expansion coefficient than
that of the first members 22. The second members 23 are adhered
onto the side (outside) opposite the side of the first members 22
to which the piezoelectric element 6 and the rod-shaped member 25
are fixed. Each of the first members 22 and each of the second
members 23 are secured together by adhesion, welding, etc.
[0065] The rod-shaped member 25 is a piezoelectric element that is
the same as the piezoelectric element 6. The piezoelectric element
6 is connected to a voltage applying means 10 that applies a
voltage thereon, but no voltage applying means is connected to the
rod-shaped member 25. Since a piezoelectric element is employed as
the rod-shaped member 25, the thermal expansion coefficient of the
rod-shaped member 25 easily can be set equal to the thermal
expansion coefficient of the piezoelectric element 6.
[0066] The optical fiber 4 is adhered to the ends of the first
members 22 with an adhesive. Specifically, the optical fiber 4 is
fixed with an adhesive at both sides of the diffraction grating
portion 8 to one end of the respective first members 22 in a state
such that a predetermined tension is given to the diffraction
grating portion 8 while the center wavelength of diffraction
grating portion 8 is monitored. When the optical fiber 4 is fixed
in this manner, the first members 22 and the diffraction grating
portion 8 of the optical fiber 4 are heated to a predetermined
temperature: a curing temperature when the adhesive is a
heat-curable resin, and when the adhesive is an ultraviolet ray
curable resin, a temperature at which the viscosity of the resin
becomes sufficiently low to give good wettability to the resin. By
fitting the optical fiber 4 to the first members 22 in this way,
the tension applied to the optical fiber 4 is maintained at a
suitable value in a operating temperature range (-45.degree. C. to
80.degree. C.) of the optical device 1. Consequently, it is
possible to maintain the stability of the center wavelength of
reflection against the temperature variation at the diffraction
grating portion 8.
[0067] In the optical device 21 having the above-mentioned
structure, since the second members 23 are made of a material
having a thermal expansion coefficient larger than that of the
first members 22, each first member 22 and each second member 23
form a so-called bimetal structure, and consequently the first
members 22 warp to a predetermined direction according to the
variation of temperature. As shown in FIG. 7(a), at a low
temperature (e.g., 20.degree. C.) the piezoelectric element 6 and
the rod-shaped member 25 shrink similarly and tend to narrow the
interval between the first members 22. However, the first members
22 warp toward the outside (the side to which each second member 23
is adhered), and accordingly the interval between the ends of the
first members 22 (i.e., the distance between the two points, where
the optical fiber 4 is fixed at both sides of the diffraction
grating portion 8) expands.
[0068] On the other hand, at a high temperature (e.g., 70.degree.
C.) the interval (the distance between the two points, where the
optical fiber 4 is fixed at both sides of the diffraction grating
portion 8) between the ends of the first members 22 narrows
because, as shown in FIG. 7(b), the first members 22 warp toward
the inner side (which is opposite the side on which each second
member 23 is fixed), although the piezoelectric element 6 and the
rod-shaped member 25 expand similarly and tend to expand the
interval of the first members 22. The illustration of the voltage
applying means 10 is omitted in FIGS. 7(a) and (b).
[0069] In the optical device 21 having the above-mentioned
structure, when a voltage is applied to the piezoelectric element 6
from the voltage applying means 10, a displacement occurs in the
longitudinal direction of the piezoelectric element 6 according to
the applied voltage, and the displacement is enlarged by the first
members 22 as shown in FIG. 8, and is transferred to the
diffraction grating portion 8. Therefore, the center wavelength of
reflection at the diffraction grating portion 8 can be varied
efficiently according to the magnitude of voltage applied to the
piezoelectric element 6.
[0070] Thus, in the optical device 21, the temperature dependence
of the center wavelength of reflection can be substantially
eliminated because the tension applied to the optical fiber 4 is
decreased, and the center wavelength of reflection at the
diffraction grating portion 8 becomes approximately .lambda.b (nm)
since the interval between the ends of the first members 22 narrows
to a predetermined value G6 (<G5) as shown in FIG. 9(b) when the
ambient temperature of the optical device 21 rises to 70.degree.
C., for example, under the condition that the interval between the
ends of the first members 22 becomes a predetermined value G5 as
shown in FIG. 9(a), and the center wavelength of reflection at the
diffraction grating portion 8 becomes .lambda.b (nm) when no
voltage is applied to the piezoelectric element 6 at normal
temperature (e.g., 20.degree. C.). Accordingly, under a condition
(the condition shown in FIG. 9(c)) in which a voltage is applied to
the piezoelectric element 6, the interval between the ends of the
first members 22 changes only to an extent that corresponds to the
displacement of the piezoelectric element 6, and the center
wavelength of reflection at the diffraction grating portion 8
changes only to an extent (.DELTA..lambda.) that corresponds to the
applied voltage and becomes .lambda.b+.DELTA..lambda.(nm).
[0071] Also, the temperature dependence of the center wavelength of
reflection can be substantially eliminated because the tension
applied to the optical fiber 4 is increased, and the center
wavelength of reflection at the diffraction grating portion 8
becomes approximately .lambda.b (nm) since the interval between the
ends of the first members 22 expands as described above when the
ambient temperature of the optical device 21 decreases to
-20.degree. C. from a normal temperature (e.g., 20.degree. C.)
under the condition that no voltage is applied to the piezoelectric
element 6. Accordingly, under the condition in which a voltage is
applied to the piezoelectric element 6, the interval between the
ends of the first members 22 changes only to an extent that
corresponds to the displacement of the piezoelectric element 6, and
the center wavelength of reflection at the diffraction grating
portion 8 changes only to an extent (.DELTA..lambda.) that
corresponds to the applied voltage and becomes
.lambda.b+.DELTA..lambda.(nm).
[0072] As described above, in the optical device 21 of the present
embodiment, the interval between the ends of the first members 22
changes as if the first members 22 apparently had a negative
thermal expansion coefficient, and hence, as in the case of the
first embodiment, it is possible to restrain the dependence of the
center wavelength of reflection at the diffraction grating portion
8 upon the voltage applied to the piezoelectric element from
shifting to the long wavelength side or the short wavelength side
according to the variation of temperature. In the optical device 21
since the mechanism for generating a negative thermal expansion
coefficient by the first members 22 and the second members 23 and
the mechanism for expanding the displacement of the piezoelectric
element are independent of each other, it is possible to set a
large amount of wavelength shift.
[0073] When the first members 22 are made of Invar alloy, a design
can be made such that the first members exhibit a large amount of
warp because the thermal expansion coefficient of Invar alloy is
low. Also, when the first members 22 are made of ceramics, the
first members can be designed to exhibit a large amount of warp
because the thermal expansion coefficient of the ceramics is low.
When the second members 23 are made of aluminum alloy, a large
amount of warp of the first members can be designed because the
thermal expansion coefficient of aluminum alloy is high.
[0074] (Third Embodiment)
[0075] An optical device according to the third embodiment of the
present invention comprises an optical fiber having a diffraction
grating portion formed in a given length along the optical axis
thereof, a rod-shaped piezoelectric element, a voltage applying
means for applying a voltage to the rod-shaped pie zoelectric
element, a rod-shaped member, first members forming a pair of arm
parts, and second members adhered to the first members. The optical
fiber is fixed to an end of the respective arm parts of the first
members in a manner that the diffraction grating portion of the
optical fiber is positioned between the ends of the arm parts of
the first members as if the ends of the arm parts stride the
diffraction grating portion. The rod-shaped member is fixed to the
other ends (i.e., opposite the ends to which the optical fiber is
fixed) of the first members constituting the pair of arm parts such
that the rod-shaped member and the pair of arm parts of the first
members form a U-shaped member. The piezoelectric element is fixed
to the arm parts of the first members such that the piezoelectric
element is connected to the intermediate points of the arm parts.
The rod-shaped member and the piezoelectric element have a
substantially equal thermal expansion coefficient. The second
members are adhered to the arm parts of the first members
longitudinally on the side to which the piezoelectric element and
the rod-shaped member are fixed. The second members are made of a
material having a thermal expansion coefficient that is lower than
that of the first members. The first members can be made of
aluminum and the second members can be made of negative thermal
expansion material such as glass-ceramics (.beta.-eucryptite).
[0076] In the optical device according to the third embodiment of
the present invention, the interval between the ends of the pair of
arm parts of the first members where the optical fiber is fixed
changes according to the variation of temperature in the following
manner.
[0077] When the temperature becomes low, the rod-shaped member and
the piezoelectric element shrink and tend to narrow the interval of
the pair of arm parts of the first members. However, since the
first members shrink more than the second members, the first
members are transformed into an arc-shape, in which the side where
each second member is attached is the outside of the arc.
Accordingly, the interval between the ends of the pair of arm parts
of the first members is enlarged.
[0078] On the other hand, when the temperature becomes high, the
rod-shaped member and the piezoelectric element expand and tend to
expand the interval of the pair of arm parts of the first members.
However, the first members expand more than the second members, and
the first members are transformed into an arc-shape, in which the
side where each second member is attached is the inside of the arc.
Accordingly, the interval between the ends of the pair of arm parts
of the first members is narrowed.
[0079] As described above, the interval between the ends of the
pair of arm parts of the first members changes as if the first
members apparently had a negative thermal expansion coefficient. As
a result, it is possible to restrain the dependence of the center
wavelength of reflection at the diffraction grating upon the
voltage applied to the piezoelectric element from being shifted to
the long wavelength side or the short wavelength side according to
the variation of temperature. Also, the displacement caused to the
piezoelectric element can be enlarged and transferred to the
diffraction grating by the first members and the rod-shaped
member.
[0080] Moreover, since the mechanism for enabling the first members
to behave as if virtually they had a negative thermal expansion
coefficient and the mechanism for enlarging the displacement of the
piezoelectric element are independent of each other, a degree of
freedom is provided for designing the appropriate variation to be
given to the center wavelength of reflection at the grating
portion.
[0081] Although three embodiments of the present invention are
described in detail above, the present invention is not limited to
them. For example, the rod-shaped member 25, which is a
piezoelectric element in the above embodiments, may be a member
made of other materials, such as ceramics, nickel copper or glass,
having a thermal expansion coefficient equal to the piezoelectric
element.
* * * * *